The brain scan in the Walking Dead scene shows that there is just enough electrical activity to stimulate the hypothalamus. Additionally, the activity from the brain scan also shows stimulation in the limbic system, which houses the amygdala. Finally, note that the activity does not travel very far through the cerebral cortex, particularly the frontal lobes (the seat of civilization), where judgment occurs. Given what you know about hormones, the limbic system, amygdala, and frontal lobes, explain the zombies' hunger, aggression, and lack of inhibition.